Did my third autocross today and have a question on air pressure. According to the sticker on my car, my front should be 35 and back 45. Of course I rarely actually have that in them but what should I run for autocross? I have regular all season tires from Kumo.

Starting to think I should be making some changes to my R32 if I want to continue with autocross but not sure what I can do that does not require tons of money. Will eventually get a second set of tires but anything else I could do (with my dads help).

check this out Erika, after you read it put it in practice. It helped me put a lot to understand how to adjust tire pressures depending on track temperature, layout of the course and my own driving skills... I'm running class A currently and this is the only way I have to adjust the suspension since I have the stock factory springs an struts.

If you feel that you need more help, just ask, we'll be glad to help.

...
Tire Pressure Woes – Autocross Tuning Tips
Posted on May 27th, 2010 in Racing Ready
Hello and welcome to Racing Ready! If you are new here, you might want to subscribe to the RSS feed.
Thanks for visiting & come back soon for more updates to Racing Ready !
Steven Johnson is a serious autocrosser. He has learned much as an HPDE (High Performance Driver Education event) participant. He also serves as the coordinator of the Harris Hill Road Chrono-X Time Trial Series. In addition, Steven is looking for a different career direction.
In the meantime, he asked me if he could share his knowledge & observations with the Racing Ready community. Knowing how thorough a competitor & helpful instructor Steven is, I did not hesitate to welcome him as Racing Ready’s newest guest blogger.
Here is his first entry…thanks Steven!

Anyone who tells you that tire pressures are not important has obviously never seen the limits of their own car. When I first started autocross, I would simply add pressure and hope for the best. How naive was I? As I found out at the last SPOKES/SASCA autocross, tire pressures can mean the difference between a smooth run and absolute chaos! My rear pressure used to be too low, giving me too much rollover. Then I tried too much pressure and the car was drifting all over the place. It was so bad that I wasted 3 runs just because I added 5 psi. After that day, I learned to respect the pressure of your tires.

I run a set of Dunlop Direzza Starspec Z1s in the STR class. The class allows for a 140 treadwear with a 255 width maximum. Unfortunately, the Starspecs are not considered the best tire for the class. The Toyo R1-R at a 140 treadwear seems to be the tire of choice. So why did I choose the Dunlop? Is it because I like a challenge? Maybe because I enjoy loosing? No, the reason is because the autocross gods frown upon heavy cars like my 350Z. My car requires a larger and taller tire than the Miata’s, Mr2′s, and S2000′s.

Simply put:
• The Toyo R1-R is better in a smaller, shorter tire
• The Dunlop Direzza is better in a larger, taller tire
So while smaller cars can run the best tire in the class, I am forced to use the underdog Direzza. That being said, I love these tires! They stick great and heat up quickly. In fact, if you ask some drivers they will tell you that treadwear doesn’t tell the whole story. They will tell you that the Direzza is just as good, if not better, than the R1-R and it depends on the course conditions. For instance, the Direzza is said to have a little bit better dry course handling. In the rain, the R1-R cannot be touched. So which tire should you choose? Don’t ask me, ask your car. How heavy is it, what tire size does it need, and what type of driving do you plan on doing?

Back to tire pressure:
What difference can a couple of psi make? More than you would think! I used to just guess at what was right and put in around 28-30 psi front and rear. I would just go out with whatever handling characteristics it gave me and deal with it. Then if the pressure jumped up a few pounds (which it will if you drive right) I would bleed off enough to get back to where I started. This is the WRONG way to setup a car’s tire pressures. I asked around and found out a few key tips to setting tire pressure:
1. You can use tire pressure to actually add or subtract traction in front or rear
2. More pressure = less traction Less pressure = more power can be transferred
3. Finding the right balance between front and rear make a huge difference
4. You can induce understeer just by having more pressure in the front than in the back
5. Don’t be afraid to have a 10 psi spread to find out what works.
I found out that my front tires actually worked best if the tire pressures were pumped up very high to around 38-40 psi. My rear tires at that pressure couldn’t get traction AT ALL! I ended up with the rear pressure at 32-33 which gave me the balance I was looking for. I thought that there was some rule out there that said don’t let your pressure be over 5 psi different from front to rear. As tip #5 above states: Don’t be afraid to have a dramatic difference from front to rear. It can teach you a lot if you are not afraid to try out extremes.

Marking your tires is a great way to gauge what pressures to use. I use a Slime tire marker from AutoZone (see picture above).

Make three marks equidistant on the outside of the tire. If you look closely at your tires, you will find a small triangle shape at the edge of the tread. This is to help you in finding your perfect pressure, so use the tire marker here. You should mark your tire next to these triangles from the top of the tread down to the sidewall.

After your run, the marker will rub off to a certain place. You want the marker to be rubbed off at the tip of the triangle for optimum pressure. If there is still markings above the triangle, your pressure is too high. If the markings are rubbed off past the tip of the triangle, your pressure is too low. You are looking for the maximum usage of tread without rolling over into the side wall.
I could go on all day about tire pressure, but you won’t know what works best for your car until you get out there and play with the extremes. You will be happy you did.

That info I postet the other day was found in the internet by me while I was researching several topics about autocrossing and I tought it was interesting. Now that Erika was asking about tire pressure I tought it wold be a good idea to share that information.

Erika:

I know how you feel, and I also hope that eventually have a nice "R" compuond set of tires that I can swap before and after the event just like the other guys. Other than te "R" tires is not worth to spend time on swaping them uot, in my opinion!!!

I currently run my unexpensive daily street tires (General Tire) I was thinking that I could have a dedidated set of tires just to be used for autocrossing but that is just a waist of time I think, I rather mess one or the tires I run now and if it needs to be replaced I just do that.

I've learned a lot of good things follwing the advise I found on that site and also from other autocrossing books and I ca tell you guys I does help a lot to understand a little bit more about suspension dinamics, understanding the course layout, tire pressures and temperature, track temperature and all that stuff, in the end it does makes you a better driver if you can undestand what that the tires and the whole suspension is telling you, and when you find the right air pressure and the right ballance all arround the car you will be a faster driver. There was a time when I was out there at an event and had different tire pressures on al four corners and was able to shave about two seconds or so from the previous run.

So my advise is to go all out and try to understand what is happening at hte same time....

That info I postet the other day was found in the internet by me while I was researching several topics about autocrossing and I tought it was interesting. Now that Erika was asking about tire pressure I tought it wold be a good idea to share that information.

Erika:

I know how you feel, and I also hope that eventually have a nice "R" compuond set of tires that I can swap before and after the event just like the other guys. Other than te "R" tires is not worth to spend time on swaping them uot, in my opinion!!!

I currently run my unexpensive daily street tires (General Tire) I was thinking that I could have a dedidated set of tires just to be used for autocrossing but that is just a waist of time I think, I rather mess one or the tires I run now and if it needs to be replaced I just do that.

I've learned a lot of good things follwing the advise I found on that site and also from other autocrossing books and I ca tell you guys I does help a lot to understand a little bit more about suspension dinamics, understanding the course layout, tire pressures and temperature, track temperature and all that stuff, in the end it does makes you a better driver if you can undestand what that the tires and the whole suspension is telling you, and when you find the right air pressure and the right ballance all arround the car you will be a faster driver. There was a time when I was out there at an event and had different tire pressures on al four corners and was able to shave about two seconds or so from the previous run.

So my advise is to go all out and try to understand what is happening at hte same time....

I'll tell you now there is no point in getting R compound tires for auto-x. I have been running them this season, and unless you can set them up properly with the right amount of negative camber and be able to get them up to running temps 160-220 degrees F they're a pain in the butt. Yes they still do wonders not at optimal conditions but there are twice as many more tires out there that will be able to give you the same results all the while be able to daily drive them. Here are a list of a few:

All of these tires you'll be able to autocross with and post really quick times with and still manage to get around 20k miles out of them...possibly 40k with the Michelin PSS's. I'll tell you a nice set of wheels that are strong and lightweight paired with one of the above mentioned tires will go a long way.

The two biggest and most helpful "mods" one can do to any car for autocross is: the Driver Mod and a proper wheel and tire setup.